(1) Field of the Invention
The present invention relates to a food drying process using a low humidity drying gas.
(2) Description of the Prior Art
The hot air drying system utilizing the temperature difference as a drying promotion factor or the freeze drying system are ordinarily adopted for drying foods.
According to the freeze drying method, since a food is frozen while maintained in vacuo and its frozen water is sublimated to effect drying, extremely high vacuum is indispensably required to enhance the drying speed. Accordingly, equipment expenses and power consumption are increased. Moreover, since transmission of heat necessary for ice water sublimation has to be accomplished by heat conduction through a heat conducting plate, the food is rendered porous and the heat conductivity is reduced with advancement of the dried food zone, and therefore, the drying speed is drastically reduced and a long time is necessary for completion of drying. Still further, since the cell structure of the food is destroyed by freezing, the food becomes brittle, the structure of the food after restoration is weak and the flavor or taste of the food is inevitably depleted or changed by the drying operation.
By the hot air drying method utilizing the temperature difference as a drying promotion factor, since a large quantity of hot air shall be in contact with a food, unsaturated fatty acids and other easily oxidizable substances, which are present in the surface portion of the food, are oxidized to cause discoloration and deterioration. Since a food is composed of living bodies, if drying is carried out at a temperature higher than 30.degree. C. over a period of a long time, the activity of enzymes may become vigorous to cause deterioration of the food. If drying is conducted at a temperature higher than 60.degree. C., such undesirable phenomena as coagulation of proteins and case hardening of a food may take place.
As food drying means to overcome the foregoing disadvantages, in Japanese patent publication No. 25854/65 there is proposed a method for drying foods by utilizing the humidity difference as a drying promotion factor, in which a low humidity nitrogen gas separated from air by cryogenic equipment is utilized for drying food and the outgoing wet nitrogen gas is not recovered, but spent away. This system is, needless to say, uneconomical.